Detect Text Box Overflow

I had roughly this problem recently with Unity 4.6 UI; hopefully you've already solved yours or work-arounded it, but to answer the question:

     private Text text;
     private RectTransform parentRect;
     [SerializeField]
     private float longestCharWidth = 50;
 
     protected void Start()
     {
         parentRect = GetComponent<RectTransform>();
         InputField inputField = gameObject.GetComponent<InputField>();
         text = inputField.textComponent;
     }

     //Return whether the text is too wide for the current field.  
     protected bool CheckTextWidth()
     {
         float textWidth = LayoutUtility.GetPreferredWidth(text.rectTransform); //This is the width the text would LIKE to be
         float parentWidth = parentRect.rect.width; //This is the actual width of the text's parent container
         return (textWidth > (parentWidth-longestCharWidth)); //is the text almost too wide?  Stop when the next character could be too wide
     }
 

Specifically, my use case was to make sure a user couldn't type a too long entry into an InputField. Since the InputField would scroll over/resize once it overflowed, I had to subtract the longestCharWidth.

The main part of my answer comes from: http://answers.unity3d.com/questions/836642/new-ui-46-how-to-show-ellipsis-for-text-overflow.html

And here's my full solution (to making sure a user couldn't enter too much info)

     public char ValidateInput(string text, int charIndex, char addedChar)
     {
         if(CheckTextWidth())
         {
             return '\0';
         }
         else
         {
             return addedChar; 
         }
     }

look at TextGenerator and all the fun stuff with it... you can get the length of what would be rendered then compare with your dimensions

In case anyone else lands here looking for help with this, I wrote an extension to the Text class. I haven't tested the one for width yet but the verticle test definitely works

 using UnityEngine;
 using UnityEngine.UI;
 
 public static class TextExtension
 {
     /// <summary>
     /// Returns true when the Text object contains more lines of text than will fit in the text container vertically
     /// </summary>
     /// <returns></returns>
     public static bool IsOverflowingVerticle(this Text text)
     {
         return LayoutUtility.GetPreferredHeight(text.rectTransform) > GetCalculatedPermissibleHeight(text);
     }
 
     private static float GetCalculatedPermissibleHeight(Text text)
     {
         if (cachedCalculatedPermissibleHeight != -1) return cachedCalculatedPermissibleHeight;
 
         cachedCalculatedPermissibleHeight = text.gameObject.GetComponent<RectTransform>().rect.height;
         return cachedCalculatedPermissibleHeight;
     }
     private static float cachedCalculatedPermissibleHeight = -1;
 
     /// <summary>
     /// Returns true when the Text object contains more character than will fit in the text container horizontally
     /// </summary>
     /// <returns></returns>
     public static bool IsOverflowingHorizontal(this Text text)
     {
         return LayoutUtility.GetPreferredWidth(text.rectTransform) > GetCalculatedPermissibleHeight(text);
     }
 
     private static float GetCalculatedPermissibleWidth(Text text)
     {
         if (cachedCalculatedPermissiblWidth != -1) return cachedCalculatedPermissiblWidth;
 
         cachedCalculatedPermissiblWidth = text.gameObject.GetComponent<RectTransform>().rect.width;
         return cachedCalculatedPermissiblWidth;
     }
     private static float cachedCalculatedPermissiblWidth = -1;
 
 }

If you aren't familiar with using extensions, you create a TextExtension script in your project, then you can call this from any other script like it is built-in Text method.

 // set in inspector
 public Text myText;
 
 private void Update()
 {
    print("text is overflowing? " + myText.IsOverflowingVerticle();
 }